Microelectronic devices are fabricated on a semiconductor substrate as integrated circuits in which various conductive layers are interconnected with one another to permit electronic signals to propagate within the device. An example of such a device is a complementary metal-oxide-semiconductor (CMOS) field effect transistor (FET).
A gate electrode is part of an integrated circuit. For example, a CMOS transistor comprises a gate structure disposed between source and drain regions that are formed in the semiconductor substrate. The gate structure generally comprises a gate electrode and a gate dielectric. The gate electrode is disposed over the gate dielectric to control a flow of charge carriers in a channel region that is formed between drain and source regions beneath the gate dielectric. The gate dielectric typically comprises a thin (for example, 10 to 50 Angstroms) material layer having a dielectric constant of about 4.0 or greater (for example, silicon dioxide (SiO2), silicon oxynitride (SiON), hafnium dioxide (HfO2), and the like). As the gate length of silicon CMOS devices is scaled to less than 100 nm, new high dielectric constant (K) materials will likely replace silicon oxide. In addition, metal gates will likely replace polycrystalline silicon (polysilicon) gates. For example, in some CMOS transistors, the gate electrode may be formed from at least one of a metal (e.g., titanium (Ti), tantalum (Ta), tungsten (W), and the like) and metal-containing conductive compound (e.g., titanium nitride (TiN), tantalum nitride (TaN), tungsten nitride (WN), and the like). Replacement of polysilicon as a traditional material of the gate electrode with metals and metal-containing compounds reduces undesired voltage drops associated with the polysilicon depletion effect, as well as increases drive current performance and the operational speed of the CMOS transistor.
As mentioned above, alternative materials have been investigated as replacements for polysilicon gates. In addition, alternative structures have been investigated as replacements for polysilicon gates. Examples of such structures include bilayer structures made from conductive metals with different work function values. These structures, which will be referred to herein as bilayer gates or sandwich gates, include a gate dielectric formed on a substrate, with a first metal layer and a second metal layer formed over the gate dielectric. Although improvements to semiconductor gate electrodes have been made through the use of alternative gate structures and materials, further improvements are desired to improve the performance of integrated circuit devices.